There are a great variety of wireless communication systems being used in the world today, such as Global System for Mobile Communication (GSM), Digital Advanced Mobile Phone Service (DAMPS), Personal Handy System (PHS), Personal Communication Services (PCS), Land Mobile Radio (LMR), Special Mobile Radio (SMR) and two way paging to name a few. The consumer would like their phone or pager to reliably work anywhere in the world, with any system architecture without costing them a small fortune. This desire has led mobile equipment manufacturers to offer dual mode and dual band models. Dual mode/band capabilities can accommodate the different modulation and data formatting protocols, as well as accommodate different frequencies required in various parts of the world. These features could allow the consumer to use a single phone in Europe (GSM) and the United States (PCS), a desirable situation in an increasingly smaller world.
Typically, the Voltage Controlled Oscillator (VCO) generates the Local Oscillator (LO) signal for transmit and receive functions in mobile radio RF circuitry. In dual band radios, a single VCO is desired to operate at two different frequencies to meet the requirements of two bands.
Prior art dual band devices have been implemented in the following ways: a single oscillator followed by a multiplier circuit, two separate oscillators with a switching network after the devices to select the appropriate oscillator, extremely wide band oscillators that operate over the entire frequency range, a dual band oscillator with commonly connected emitters in which the bias is changed to turn on a first oscillator at a frequency then to transition to a second frequency as the second desired oscillation frequency, or two completely separate Voltage Controlled Oscillators (VCOs) that are matched then combined using a combiner network such as a Wilkinson Power Divider.
The disadvantage of using a single oscillator with a multiplier circuit following the oscillator is the multiplier circuit requires current to function and the current drain may decrease the battery life of the device when used in a portable application. There are also harmonics of the fundamental frequency generated in addition to the desired frequency and these unwanted spurious signals may be a problem in the system and require extra filtering circuitry which will add cost and complexity.
The disadvantage of using two oscillators with an output switching network is each oscillator must be matched to the reference impedance of the system, typically 50 ohms for RF circuits in this frequency range, then the switching circuit must be implemented again adding complexity and parts to the device.
The disadvantage of using an extremely wide band oscillator is that to accomplish the wide tuning range a large capacitance change is required in the tank circuit. This large capacitance change requires a large voltage change applied to the variable capacitor (a varactor diode) which is unavailable in many applications and the circuit Q is degraded which increases the noise in the circuit rendering the oscillator unable to meet the phase noise requirements of the application.
The disadvantage of using the dual band oscillator with the common coupled emitters is the output is typically taken from a common point between the two collectors or emitters which are coupled. This was disclosed by Reeser et al. in U.S. Pat. No. 5,856,763. This requires a matching network be synthesized to match the transistor output to the reference impedance of the system, again typically 50 ohms at these frequencies. This network may be complex since the network will pass the lower frequency and the higher frequency. Since the higher frequency is approximately twice the first frequency, a wide band output network should be used, and the second harmonic of the lower frequency oscillator will be within the passband and may be passed to the output creating unwanted spurious signal.
The disadvantage of using two different oscillators with a Wilkinson combiner is each individual oscillator must be matched to the reference impedance of the system, again typically 50 ohms in RF circuits, then the Wilkinson combiner is implemented. The Wilkinson combiner will be unbalanced when either oscillator is shut off providing unequal loads on the input of the combiner. This will alter the amplitude response of the combiner. One solution to this dilemma is to provide an amplifier to the output of the oscillator before the combiner. This results in extra current and circuitry requirements.
There are switched resonator designs that switch the a resonating element in the tank circuit to achieve a difference in the operating frequency of the device. This is typically achieved by switching either an extra capacitance into the tank circuit or switching an amount of inductance into the tank circuit. These approaches allow the frequency of the oscillator to be altered by approximately 10 to 40%. The reason for this is the amount the inductance or capacitance in the tank circuit can be altered and have adequate gain to sustain oscillation is limited.